1. Background of the Invention
The present invention generally relates to a light amplifier device capable of suppressing a light surge.
A light amplifier device is known in which a signal light and an exciting light are applied to an optical fiber doped with a rare-earth element such as erbium (Er). Such an amplifier is capable of amplifying the signal light per se, and is thus applied to a repeater amplifier in a long-distance light transmission system. In the light amplifier device using a rare-earth element doped optical fiber, it is known that a light surge occurs when the light signal is recovered immediately after the light signal is broken down. Such a light surge may damage optical circuit components in parts of the subsequent stages. Hence, it is desired to suppress the light surge.
2. Description of the Related Art
Generally, the light amplifier device uses an erbium-doped optical fiber which amplifies a light signal in the 1.5 .mu.m band. Also, there are known light amplifier devices that utilize a neodymium-doped (Nd-doped) optical fiber capable of amplifying a light in the 1.06 .mu.m or 1.32 .mu.m band, and a praseodymium-doped (Pr-doped) optical fiber capable of amplifying a light in the 1.3 .mu.m band. Generally, the gain of the light amplifier device is controlled by controlling the power of the exciting light. Alternatively, the light amplifier device having an optical fiber as described above has a saturation characteristic, and therefore the gain thereof can be controlled with the power of the exciting light kept constant.
As described above, a light surge occurs when the light signal is recovered immediately after the light signal is broken down, for example, after a short period of approximately 10 .mu.s. Such a short-period breakdown results from, for example, an instantaneous breakdown of an electric signal on the transmit side, an inserting and detaching step of an optical connector, or bending of the optical fiber with a small radius of curvature. When the light signal is recovered after the short-period breakdown, energy of the exciting light or the like stored in the rare-earth-doped optical fiber produces a light surge. If the light surge is input to a photodiode which is a component of a light-receiving part, the photodiode may be destroyed. If an oil film is present between the interface of joined fibers in the optical connector, the optical fibers may be blackened and thus the optical connector may be damaged. If the light surge is emitted outside, humans or animals may be affected.
With the above in mind, a structure provided on the input side of the light amplifier device has been proposed in which a light component extracted from a received (input) light signal is converted into an electric signal, which is monitored in order to determine whether the input light signal is normal. If a breakdown of the input light signal is detected, the operation of a semiconductor laser which emits the exciting light is stopped. There is an alternative proposal in which when the breakdown of the input light signal is detected, a light switch is turned off in order to prevent the amplified light signal from being applied to a following stage.
Another alternative has been proposed in, for example, Japanese Laid-Open Patent Application No. 6-204947. More particularly, the wavelength of the light signal is set outside of the maximum gain wavelength band of the light amplifier device using the rare-earth element doped optical fiber. An optical filter that has a blocking band including the above maximum gain wavelength is proposed. The optical filter functions to block the light surge because the light surge occurs at the maximum gain wavelength of the light amplifier device.
However, the above related art has the following disadvantages. As described above, the occurrence of the light surge is suppressed by stopping, in response to the detection of the breakdown of the input light signal, operation of the semiconductor laser which outputs the exciting light. When the input light signal is recovered, the semiconductor laser is driven again, and thus the input light signal is amplified. However, it takes a certain time to recover the normal amplifying gain. This is not good for a long-distance light transmission system which includes a plurality of light repeaters installed at given intervals. The time necessary to totally recover the normal gain in the whole system is increased in proportion to the number of repeaters. Further, the light amplifier device does not have any means for detecting the light surge although it has the function of detecting the breakdown of the input light signal. Hence, it is impossible to prevent the light surge from being amplified and output.
In the structure that uses the light switch which prevents the light signal and the light surge from being applied to a following stage, it is required that the light switch and its driving circuit operate at a high speed. However, in practice, it is not easy to provide a light switch which has a small insertion loss and a high operation speed.
In the structure that uses the light filter, it is required that the light filter has a sharp filter response capable of cutting off the maximum gain wavelength of the light amplifier device. However, in practice, it is not easy to provide a light filter having such a sharp cutoff response.